I wonder whether this is a new focussing algorithm which Nikon has been using with it's high resolution sensors or whether it is a function of the effect of high resolution on the pdaf sensors themselves.

The D800 does the same. I came from a D7000 and so was familiar with this, avoided it and have had no focussing issues at all.

I only use dynamic for birds in flight and use AFC single point with afon for everything else.